Position sensitive detectors are used to measure spatial coordinates of incident particles thereon. Such particles can be, e.g. photons, electrons, neutrons, ions, x-rays etc. Such detectors are used to detect the particles and their position on a sensitive area of the detector. However, in order to detect individual particles, the signal resulting from that detector needs to be amplified, which may be done, e.g., by an electron multiplier arranged inside the detector. The amplification leads to a formation of an avalanche of electrons inside the detector resulting in a cloud of electrons inside the detector. From this measured cloud, the initial position of the particle incident on the detector needs to be calculated.
An electron multiplier may be formed by a micro-channel plate (MCP), which is an array consisting of 104-107 tiny channels. Each individual channel acts like a miniature electron amplifier. The technology of MCP production has been developed for the last forty years. Originally, they were developed as X-Ray detectors or ion sensors for the needs of nuclear physics experiments. The channel diameter has been improved starting from 100 μm in the 1960s to modern plates having a pore diameter of 3.2 μm. The pore or channel size and the pore-to-pore distance limit a maximal spatial resolution of the detector.
A maximal gain factor of the single MCP channel could reach up to 103-104. Such a small number of electrons can neither be easily detected nor precisely measured utilizing the current electronic components. Usually, assemblies or stacks of MCPs are used, for instance: two (chevron), three (Z-stack) or multi MCP assembly. An example of such detector is shown in FIG. 1 comprising two MCP stacked on top of each other forming a single stack. Particles are incident on a photo-cathode 3 of a detector 1. The photocathode 3 converts the initial particle into a photoelectron by the photoelectric effect. The photoelectron is then amplified through the generation and multiplication of secondary electrons while passing through the MCPs 4. When a quantum, e.g. a photoelectron in the present embodiment, to which the MCP is sensitive is incident on the inner wall of one channel, at least one electron is emitted from the inner wall. The electron emitted from the inner wall of the channel is then accelerated by an electric filed generated by a voltage applied to both ends of the MCP, and travels to collide again with the channel wall to generate a secondary electron. This process is repeated many times along each channel, so as to multiply and accelerate the electrons, such that a large number of electrons are emitted from the output face of the MCP. The two-dimensional position of the electrons is maintained by these channels. The resulting electron avalanche is finally detected on a position sensitive anode 2 of detector 1. The charge on different electrodes of the anode 2 is read-out by a read-out circuit. Each electrode produces a signal corresponding to the detection position of the initial particle.
A strong impulse for further MCP development was given by development achievements on night vision detectors. A typical night vision device combines a photocathode in front of the MCP assembly and a phosphor screen at the output. An incident photon hits out the photoelectron from the surface of the photocathode and gets amplified inside the MCP stack. A resulting electron cloud is focussed applying an electric field in the interval between the last MCP and the screen to a small point on the surface of a phosphor layer inducing a light flash intensive enough to be visible by an eye or measured by a charge coupled device (CCD) based detector. Depending on the material of the screen, the light flash decay time may vary in range of 1-50 μs. As a result, the system produces a durable flash of light at the output even for a single incident photon. This type of light detection belongs to the integrating measurement class; while it is sensitive enough to register individual quanta of light, the method does not distinguish single photons due to the long-life decay times of the resulting flashes.
In contrast, a single particle counting detector allows registering an individual event in order to acquire additional parameters along with the spatial coordinates of an incident particle. A typical counting device comprises an MCP stack and an anode system. The cathode is an optional part to convert low-energy photons into electrons. An incident particle interacting with the front surface of MCP results in an avalanche of electrons. The resulting electron cloud, following an applied electrical field, falls onto the anode, where it is detected.
A number of well-known anode systems are commonly used for various applications like single photon counting, nuclear physics experiments etc.
In general, space sensitive anodes can be separated into two domains of the space coordinate measurement principle: time and charge (current) based. The simplest example for a time-based anode is a one-dimension delay-line arrangement, in which the coordinate is measured by use of a time difference between arrival of the signal to the ends of the line. There is also a number of known charge division based anode system configurations: wedge-strip, quadrant, Vernier, resistive layer, to mention some examples.
It is clear that greater amplification factors can easily be achieved by stacking more and more MCPs on top of each other. Higher amplification will result in a better signal-to-noise ratio (SNR) and finally cause a higher spatial resolution. However, using stacks of three and more MCPs will result in a shorter lifetime of the detector due to the limited resource of the MCP (amplification degradation). Most of the delay line detectors (time domain anodes) require three or more MCPs for proper operation.
All the time based measurement methods are based on the same approach: they measure the time difference between the pulses acquired from the ends of various configurations of delay lines. Delay line based systems are widely used in nuclear physics experiments. In delay line detectors, the avalanche from the MCP stack crosses a meander-like delay line. The charge pulse induces an electric pulse on the wires of the delay line, which propagates to the two ends of the wires. Electronic time-to-digital or time-to-amplitude converters measure the difference of the arrival time of the signal at each end. The measured time difference of arrival is proportional to the respective position of the charge cloud in the direction of arrangement of the delay line, i.e. a delay line with respect to the lateral x-coordinate or a delay line with respect to the lateral y-coordinate. The delay line may be employed in single particle counting measurements.
Charge division based anodes belong to a large family of anode systems employing the principle of dividing the electron cloud charge into smaller parts. The anode comprises a planar or complex surface that can be a combination of the planar, cylindrical, cone or spherical surface parts. The surface is covered by a set of electrode elements electrically isolated from each other or combined in order to form a position sensitive pattern. A volume of an electric charge carried by the electron cloud induced by an incident particle is shared between the electrode elements.
Lampton et al, “Quadrant Anode Image Sensor”, RSI, Vol. 47, No. 11, November 1976, p. 1360 discloses a quadrant anode (QA) based image sensor as shown in FIG. 2. The anode is a metal plate divided into four electrically isolated quadrants. The quadrants are equally large. The total charge of the electron cloud is divided into four parts: left top qlt, right top qrt, left bottom qlb and right bottom qrb. Each charge value is measured. In order to estimate the initial position of the incident particle onto the detector, the difference between the sums of the charges that fell above and below the horizontal line of division of the electrodes being normalized to the value of the total charge may define the y coordinate of the incident particle. The horizontal coordinate x is calculated the same manner using left and right charge parts:x=(qrt+qrb−qlt−qlb)/qΣ,y=(qrt+qlt−qlb−qrb)/qΣ,where qΣ=qrt+qrb+qlt+qlb, i.e. the whole measured charge.
The method presented by Lampton achieves high spatial resolution down to a diameter of a single MCP channel. A short length of the inter-electrode border results in a low inter-electrode capacity that allowed a very precise charge measurement, since a strong dependence of noise of the charge sensitive amplifiers exists with respect to the input capacity. The ratio between the perimeter length to the area is low, i.e. the capacitance induced noise component will be low. The above equations result in a simple computation algorithm for the position sensitive anode being based on the principle of charge division. While the spatial resolution is high, the working area of the approach proposed by Lampton is limited to several millimetres in diameter with respect to the centre point of the quadrants. This is because the calculation method shown above results in high non-linearity outside the centre of the structure. Furthermore, it is important to notice that the method does not take into account the factor of an angular non-symmetry of the avalanche induced by the angle of the MCP channel.
In U.S. Pat. No. 4,395,636 A, a wedge-strip anode (WSA) based image sensor is disclosed. Such a structure is shown in FIG. 3. The WSA comprises a position sensitive anode 2 in the form of periodically arranged sets of interconnected anode regions, which comprise wedge and strip regions, for detecting the position of the centroid of a charge cloud arriving thereat from a charge multiplier. Each of the wedges has the same geometry and comprises a charge collection area, which varies linearly from bottom to top of the anode. A WSA employs a planar electrode pattern designed to overcome non-linearity problems described by Lampton for the QA. Due to the complex shape of the electrodes, the border becomes long. This results in a 20-30 times increased capacity in comparison to the quadrant inter-electrode capacity and, as a consequence, requires higher MCP amplification to achieve the same signal-to-noise ratio as in simpler structures. The position calculation is done by taking a linear combination of the individual charges related to the total charge. The computation method relies on a geometrical pattern of the avalanche and the assumption of a certain charge footprint.
In later work by M. Purschkea et al (“An improved quadrant anode image sensor with microchannel plates”, 1987, Nuclear Instruments and Methods in Physics Research), an advanced method for calculating the position of the centre of the electron avalanche is described. Purschkea employs a polynomial interpolation of the charge ratios used by Lampton. This is done in order to enlarge the area of linearity of a quadrant anode. To do so, it is necessary to enlarge an efficient size of the cloud footprint. In order to compensate the losses of charge on the edges of the MCPs and to collect all the charge carried by the cloud, the anode area needs to be enlarged. The combination of the larger anode area and the polynomial interpolation allowed utilizing up to 80% of the MCP area for the measurement.
J. S. Lapington et al (“Imaging achievements with the Vernier readout”, 2002, Nuclear Instruments and Methods in Physics Research) discloses Vernier anode pattern based on a planar structure of repetitive sine wave shaped stripes. The position calculation algorithm is a linear combination of the measured charges.
In A. S. Tremsin et al (“Centroiding algorithms and spatial resolution of photon counting detectors with cross-strip anodes”, Nuclear Science Symposium Conference Record, Volume 2, Issue, 2003) a combination of the avalanche footprint modelling and geometrical estimation is described. In order to compute the coordinate, Tremsin applies an estimator formula in order to find the position of the electron avalanche maximum. A large number of electrodes are used for every axis of a cross strip anode as shown in FIG. 4 in order to achieve appropriate spatial resolution. A simple average over the stripe signals is calculated to get the initial coordinate.
Further, U.S. Pat. No. 5,686,721 A discloses a charge imaging device and a method to separate a high vacuum volume of the electron amplifier from electrodes placed outside the vacuum. The major benefit of this method is a freedom to chose an anode pattern and easily change it on request. The approach is described, e.g., with respect to WSA and delay-line anode systems.
All methods and devices require a heuristic approximation for the calculation of spatial coordinates of the incident particle. A polynomial function of the measured detector responses may be used to approximate spatial coordinates as a function of the responses. The coefficients may be found from a least square equation. For example, in a charge-division based anode, the polynomial function may be a sum of products of charges, wherein the sum of the respective charge powers does not exceed a predefined integer N, the degree of the polynomial. For instance, a polynomial function for a position sensitive detector with an anode having five electrodes may be written as sum of terms like cn*q1i*q2j*q3k*q4l*q5m, where i+j+k+l+m is less or equal to N and cn is a coefficient of the n-th member of the polynomial. The number of summands is defined by the respective number of all possible and allowed combinations of non-negative numbers i, j, k, l, m. In order to approximate an unknown function of the detector responses, one has to build a polynomial having a degree high enough to precisely approximate the behaviour of the response function. However, higher degrees of polynomials suffer the problem of computational non-stability. Such non-stability is known from the prior art and affects the least square approximation procedure and the computation of the polynomial itself. These factors make it problematic and practically impossible to employ polynomials to approximate spatial coordinates as a function of the detector responses. Furthermore, increasing the number of anode outputs will result in a decrease of the final resolution due to the computational errors in the polynomial approximation, while generally a larger number of electrodes in case of charge-division based anode should result in a higher spatial resolution.
To conclude, the position calculation of different anode systems in prior art relies on a known geometry pattern of individual electrodes and the distribution of the charge parts. Heuristic estimations are then made in order to calculate the initial coordinate of incidence on the detector.
Yet, the response of a single MCP to an incident particle depending on its energy, its kind, its spatial position of the initial hit and the angle of fall relative to the pore or channel varies in a range from 0 to 104 electrons. FIG. 5 shows the process of amplification in MCPs. The efficiency of the secondary electron emission strongly depends on a combination of the energy and the kind of initial particle. MCP's are known to be highly efficient to alpha particles and ions due to the high mass and charge. The probability to emit a number of secondary electrons as a reaction to a hit off an alpha particle is around 100%. Gamma-quanta and electrons may result in an avalanche, if and only if, the energy agrees with the sensitivity spectra of the MCP for an electron.
The trajectory of the initial particle results in a varying number of secondary electrons. If the particle does not hit the sensitive surface of the channel on a first third measured from the front face of the MCP, the event will most probably be lost. This effect has two major reasons. First, if the incident particle is required to hit out a sufficient number of electrons within the first interaction act, the length of the channel will limit the avalanche development. The second reason relies on the amplification of secondary electrons. These electrons to be multiplied shall have sufficient energy to hit out more than one electron per each electron-wall interaction act. In case that an initial hit took place close to the output of the MCP channel, the electrons most probably will not be accelerated to the energies required for secondary emission.
A spatial position relative to the pore or channel edge and energy of the incident particle may cause two chains of events. The first and simplest one is that a particle will not result in any secondary amplification and will be lost.
Gamma-quanta may result a local temperature increase, scattering or hitting out an electron that potentially could result in an avalanche. In case of charged particles, scattering and electron hitting-out is possible. If one or more secondary electrons are not generated, an initial particle will not result in an avalanche and, as a result, will not be registered by the detector.
The electron emitting reaction branches may cause several effects. Depending on penetration properties of an incident particle, the depth from the surface where the electrons are generated will differ. In case of deep penetration of the initial particle, the most probable scenario will be a reunification with an ion or an atom from which electrons have been hit out. The electrons that are born close to the surface or on the surface may be captured by the channel and, probably, result in an avalanche.
Summarising the above points, the conclusion is that electron multiplication, taking place inside the microchannel or micro-sphere plate pore, is a stochastic process. A final number of electrons carried by an avalanche, see FIG. 6, depends on the following factors: kind of particle, energy range, and type of MCP. Due to the complexity and multi-branching nature of the electron multiplication, a direct measurement should be performed in order to get the information about amplification properties of the MCP.
Due to the random number of electrons carried by the avalanche, the amplitude of the signal will differ for the same excitation position. In other words, the measured volume of charge will differ even if an incident particles initially interacting with the same channel of the front MCP of the stack as shown in FIG. 7. Every dot on the plot represents a single event induced by irradiating a small spatial area of the front MCP.
Horizontal and vertical coordinates are the values of the measured charges acquired from the first and the second electrodes of the quadrant anode (see FIG. 2).
The second effect relates to position sensitive anodes based on a charge division principle as one example of the possible number of different position sensitive detectors. Depending on the size of the electron cloud, the forces of the interaction inside will differ due to the difference in the electric field inside the plasma cloud. In other words, the shape of avalanche will differ depending on the number of electrons carried by the cloud.
In general, the behaviour of the electron cloud is a dynamic and complex process that depends on a large number of parameters.
The first is the amplification properties of an MCP. Depending on the material they are made of, the amplification may vary in a wide range. Individual channel diameter may also influence the shape of the resulting cloud and a maximal number of electrons that can be produced by a single amplification act.
The second parameter is the efficient shape of the avalanche. The word “efficient” refers to a footprint of the avalanche on the anode plate where most of the electrons are located. The diameter of the charge cloud can be varied by the applied voltage between the output of the last MCP in the stack and the anode. A higher voltage will accelerate the cloud thus minimizing its time of flight from the MCP to the anode. This results in a smaller avalanche footprint diameter due to the shorter time span in which internal forces inside the cloud are applied to the individual electrons carrying charge expanding the cloud.
The third parameter concerns the asymmetry of the whole cloud induced by the angle between a central axis of the individual channel and the plate of the anode. This factor also tightly connects to the voltage in the MCP-anode gap. Lower voltages will allow the cloud to spread more along the anode and an overall distribution will be smoother. In other words, the initial non-symmetry will be blurred along the anode area resulting in a more homogeneous distribution. In an extreme case of zero applied voltage, the cloud will explode by the internal electrical field and only small outer parts of the charge may reach the anode and cover it homogeneously.
Concluding the above points, it is difficult to find a static or heuristic model of the anode or electron avalanche described in the prior art.
In addition, along with position information, a time resolution of the detection is usually desired. Time and position resolved measurement apparatuses are known in the prior art for performing time-resolved measurements of light emission to acquire a position and time of the emission. Such apparatuses employ microchannel plates (MCP) to achieve amplification with ultra-fast response times for an incident particle. A microchannel plate is a planar structure consisting of millions of miniature tubes oriented parallel to each other. Every channel acts like a miniature electron multiplier. Typically, the plate is about a millimeter or less thick and the channel diameter varies in range of 3 to 10 microns in modern plates. MCPs are known to be sensitive to electrons, charged particles and X-Ray irradiation. For single incident quanta of irradiation, a channel produces an avalanche of 103-104 electrons. In order to increase an efficient number of the resulting electrons, i.e. achieve higher amplification, stacks of two, three or more plates are used. Typically, an electron avalanche is formed and fed out from the MCP in response to an incident quantum beam during pico- to nanoseconds. The time-jitter of the front edge of the pulse induced by the electron avalanche is known to take place on a picosecond time scale. While the avalanche leaves the MCP, a deficit of electrons appears. Therefore, a power supplier is provided to compensate the charge taken away by the avalanche. This pulse can be measured and analyzed.
US 2007/0263223 A1 disclosures a time- and position-resolved measurement apparatus with MCP multiplication device and a position sensitive anode. The timing signal pulse is read from the output surface of the MCP stack. However, the measured signal changes its polarity during time. Such polarity change requires the use of zero crossing detection techniques in order to provide an accurate time measurement. The temporal resolution achieved by US 2007/0263223 A1 is about 60 ps.